Recently, it is suggested the hybrid vehicle equipped with the engine for outputting torque by combustion of fuel and the electric motor for outputting torque by supply of electric power and is capable of traveling by transmitting the torque of the engine and the electric motor to wheels. Such hybrid vehicle is configured to drive the wheels only by the torque of the electric motor or to drive the wheels by the torque of the engine and the electric motor by controlling drive and stop of the engine and the electric motor according to an operating state thereof, and the electric motor may be driven by the electric power accumulated in a battery, and when energy of the battery decreases, the engine is driven to charge the battery.
That is to say, in the hybrid vehicle, the engine and the electric motor are provided as driving force sources, and a planetary gear for synthesizing the power of the engine and the electric motor to transmit to the wheels is provided. Specifically, an output shaft of the engine is coupled to a carrier of the planetary gear, and an output shaft of the electric motor is coupled to a ring gear of the planetary gear, and it is configured that the power is transmitted from a sprocket coupled to the ring gear to the wheels. In addition, an electric generator is provided between the planetary gear and the engine and a rotational shaft of the electric generator is coupled to a sun gear of the planetary gear. Therefore, the power of the engine is divided to the wheels and the electric generator by the planetary gear, so that a rotational speed of the engine may be controlled by controlling a rotational speed of the electric generator. That is to say, a power dividing mechanism composed of the planetary gear has a function to convert the rotational speed of the engine and a function to divide the power of the engine to the wheels and the electric generator.
In the hybrid vehicle thus configured, in order to improve efficiency of the engine, the engine rotational number for driver required torque is determined by using an optimal fuel consumption operation-line determined by the engine rotational number and the engine torque. Such hybrid vehicle is disclosed in the following patent document 1.
The hybrid vehicle disclosed in the patent document 1 is equipped with a hybrid drive system having a Ravigneaux-type composite planetary gear train coupled so as to be in the order of rotational speed of a first motor generator, the engine, an output gear and a second motor generator in an alignment chart, and sets a gear ratio such that an operation point of the engine is on an optimal fuel consumption line, and when one of motor rotational speeds calculated based on a gear ratio setting is judged to be overspeed, this corrects the engine rotational number to decrease the motor rotational speed.    Patent Document 1: JP-A-2004-153946